The present invention relates generally to chemical-mechanical polishing slurries for metal, preferably for use in semiconductor device planarization, memory disk polishing, (metal containing) optics polishing, and the like. More particularly, the slurries of the present invention are preferably designed to have a low static etch rate and are preferably metastable due to reversible formation of certain types of agglomerates.
Many prior art slurries tend to agglomerate over time and form hard, dense sediment, and this problem is well discussed in U.S. Pat. No. 5,527,423 to Neville, et al. (hereafter, xe2x80x9cNevillexe2x80x9d), which is hereby incorporated into this specification by reference.
The Neville patent addresses this problem by having the slurry comprise xe2x80x9ca force sufficient to repel and overcome the van der Waals forces between the particles,xe2x80x9d (a fundamental element of all the Neville claims). To date, the solution provided in the Neville patent has had little, if any, commercial success.
Applicant has found a novel solution to the problem of the Neville patent, and surprisingly, it requires the absence of xe2x80x9ca force sufficient to repel and overcome the van der Waals forces between the particlesxe2x80x9d. Hence, the present invention is quite contrary to the teachings of Neville. Applicant""s invention takes metal slurry technology to a much higher performance level than what is discussed and described in the Neville patent.
The present invention is directed to a chemical mechanical polishing slurry for polishing metal layers, comprising metal oxide particles dispersible in an aqueous medium. The particles have a surface area ranging from about 40 m2/g to about 430 m2/g, and an aggregate size distribution less than about 1.0 micron, a mean aggregate diameter less than about 0.4 micron. The slurries of the present invention will form agglomerates of sizes in excess of 0.75 microns (and in certain other embodiments, agglomerates greater than 1 micron and 1.25 microns). In a preferred embodiment of the present invention, the agglomerates will not cause unacceptable polishing defects and will generally de-agglomerate with simple agitation.
Whereas slurries in accordance with Neville are xe2x80x9ccolloidally stablexe2x80x9d, the slurries of the present invention are not xe2x80x9ccolloidally stablexe2x80x9d. Rather, the slurries of the present invention exhibit a type of metastability. When a slurry is agitated into a uniform dispersion, then placed at rest, a stable slurry (i.e., slurries in accordance with the Neville patent) will tend to stay uniformly dispersed. Perhaps a very thin line of decantant might form at the very top of the slurry after several days or so, but fundamentally the particles generally remain well dispersed throughout at least 90% of the slurry, even after being at rest for more than two weeks.
On the other hand, the metastable slurries of the present invention will immediately start to fall out of suspension when at rest. Typically within a few hours (of being at rest), a large line of decantant will tend to form at the top of the slurry. Within 48 hours (of being at rest), as much as 80% or more of the slurry particles will tend to be located in the bottom two thirds of the slurry, and after being at rest for more than two weeks, the slurries of the present invention will generally have over 80% of the slurry particles located in the bottom half of the slurry.
The slurries of the present invention are NOT unstable, but rather, a type of metastable, wherein the particles will agglomerate and fall out of suspension when the slurry is at rest, but then, will immediately de-agglomerate and redisperse with simple agitation. In comparison, an unstable slurry will NOT readily de-agglomerate and redisperse with simple agitation, because unstable slurries will form stage 2 agglomerates (stage 1 and stage 2 agglomeration is further defined below).
Agglomerates have generally been considered undesirable for polishing. However, agglomeration occurs in two stages, and Applicant has discovered that only stage 2 agglomeration causes the predominant undesirable effects upon chemical mechanical polishing performance. The metastable slurries of the present invention will generally not form stage 2 agglomerates, but rather will substantially only form stage 1 agglomerates. Unlike stage 2 agglomerates, stage 1 agglomerates will readily de-agglomerate with simple agitation (e.g., vigorous shaking of the slurry for about 5 seconds or less).
Stage 1 agglomeration involves agglomerated particles held together primarily by van der Waal forces. Stage 2 agglomeration can occur after stage 1 agglomeration, wherein the particles then fuse together over time, causing the particles to be primarily held together not by van der Waal forces, but rather covalent (or similar-type high energy) bonding between the particles. The slurries of the present invention comprise an appropriate amount of ionic species and/or other adjuvants which diminish or otherwise prevent stage 2 agglomeration.
The ionic species used in the present invention are adjusted to diminish, inhibit or otherwise disrupt any charge layer around each particle in the slurry. For example, the anionic species in the aqueous medium will interact with, diminish or otherwise disrupt any positively charged layer around any particle, and the cationic species in the aqueous medium will interact with, diminish or otherwise disrupt any negatively charged layer around any particle.
This disruption of any charge layer around each particle substantially removes or diminishes electrostatic repulsion between particles. Such diminished electrostatic repulsion de-stabilizes the slurry and enables the particles to move sufficiently close to one another to induce a van der Waals bond between the particles, thereby creating stage 1 agglomerates. Stage 1 agglomeration may also involve hydrogen bonding between particles. A critical feature of the present invention is the absence of a force sufficient to repel and overcome the van der Waals forces between the particles, and therefore the slurries of the present invention will (when at rest) readily form stage 1 agglomerates and (partially or wholly) fall out of suspension.
During agglomeration, particles are able to move sufficiently close to one another to induce van der Waals bonds, and these bonds bias the particles together. While the particle are biased together by van der Waals forces, a second stage of agglomeration can then occur. This second stage involves bridging between the particles. Bridging occurs due to the equilibrium reactions between the particle surface and the aqueous medium surrounding the particles. The surface of the particle will tend to dissolve into the aqueous medium, then precipitate onto the particle(s). When the precipitate bridges between two particles, thereby covalently bonding the particles together, this becomes stage 2 agglomeration.
For example, although alpha alumina is generally inert (i.e., tends to resist dissolving) in an aqueous medium, conventional alpha alumina has about 1 weight percent (or more) of gamma alumina. The gamma alumina is far less inert in an aqueous medium and will typically (reversibly) dissolve, creating AlO2xe2x88x92 in a basic medium and Al+3 in an acidic medium. In either case, the reaction is reversible and the ions which dissolve from the particle will re-deposit back onto the particle(s).
When van der Waal forces bias two particles together, this re-depositing (of the dissolved alumina back onto the particles) can cause bridging between the two particles. Indeed, by dissolving and re-forming, the two particles tend to slowly fuse together into a single rigid mass. Over time, the agglomerates will be so rigidly fused together that a hard dense sediment (of stage 2 agglomerates) is formed. Stage 2 agglomerates generally cannot be effectively broken down into their original particles, except by the application of high energy, e.g., milling or high shear mixing.
Applicants have discovered that only this bridging (stage 2 agglomeration) is harmful to the polishing performance of a metal slurry. Applicant has further discovered that if such bridging is inhibited or wholly prevented, dramatically improved polishing performance can occur, even if the particles undergo stage 1 agglomerating (i.e., agglomeration substantially free of bridging) due to van der Waal forces between particles. This is preferably done by adjusting the slurry chemistry to obtain the desired state, e.g., an optimal ionic strength.
Without bridging, agglomerated particles will readily de-agglomerate with minimal agitation. Indeed, van der Waal forces are extremely weak, arguably the weakest forces which can exist between two separate bodies of matter. Without bridging, these van der Waal forces (and any hydrogen bonding between the particles) are easily overcome, and any agglomeration is not detrimental to polishing. Agglomeration without bridging will generally cause a slurry to form a fluffy xe2x80x9ccloudxe2x80x9d or layer toward the bottom of a slurry container, once left undisturbed for a period of time. With only minimal agitation, the cloud of agglomerates readily breaks apart and re-disperses in the medium. Typically, vigorous shaking of the container for less than a minute (more preferably less than 30 seconds, yet more preferably less than 15 seconds and yet more preferably in less than 5 seconds) will de-agglomerate the slurries of the present invention and cause the particles to uniformly disperse within the aqueous medium.
A further critical feature of the present invention is the inhibition or prevention of stage 2 agglomeration, after stage 1 agglomeration. This is accomplished by the incorporation of appropriate ionic species or other adjuvants which inhibit the fusing of stage 1 agglomerated particles into stage 2 agglomerated particles.
In a preferred embodiment, stage 2 agglomeration is inhibited by coating particles with a surfactant or polyelectrolyte prior to incorporating the particles into a slurry system. Alternatively, the surfactant or polyelectrolyte can be incorporated onto the particles after the particles are incorporated into the slurry system. The surfactant and/or polyelectrolyte will tend to remain in close proximity to the slurry particles, thereby sterically hindering the particles from coming sufficiently close to one another to enable bridging or stage 2 agglomeration. It has been surprisingly discovered that stage 1 agglomeration can occur even in the presence of surfactant or polyelectrolyte at the surface of the particles, and that the presence of the surfactant or polyelectrolyte will keep the particles sufficiently apart to inhibit or prevent stage 2 agglomeration.
Stage 2 agglomeration can also be inhibited by the use of complexing agents which inhibit deposition or sedimentation from the aqueous medium onto the agglomerated particles. Useful complexing agents include appropriate chelating compounds, ordinary skill and experimentation may be necessary in choosing appropriate chelating agents, depending upon the type of potential sedimentation or deposition for any particular slurry system. Generally speaking, water soluble, polar organic compounds having one or more (preferably two or more) Lewis acid moieties can be advantageous as complexing agents in accordance with the present invention. Preferred complexing agents include multifunctional acid or acid-hydroxide, water soluble organic compounds, such as, citric acid.
Stage 2 agglomeration can also be inhibited by modifying the solubility of xe2x80x9cpotential bridgingxe2x80x9d materials in the slurry (material in the slurry which is capable of deposition or sedimentation). Possible modifications may include pH modification, temperature modification, ionic strength modification and the like. Ordinary skill and experimentation may be necessary to determine the appropriate modification, depending upon the particular slurry system selected.
Sonification is a method that can be used to determine whether agglomerates are stage 1 agglomerates (stage 1 agglomerates are agglomerates which are held together primarily only by Van der Waal forces, e.g., no bridging) or stage 2 agglomerates (stage 2 agglomerates are agglomerates which are held together by Van der Waal forces and also by bridging). Generally speaking conventional, low energy sonification will break up stage 1 agglomerates but not stage 2 agglomerates. Any agglomeration of the present invention (due to the slurry being at rest for a period of time, e.g., 2 hours or more) is principally stage 1 agglomeration. Hence, the agglomerated particles of the present invention are de-agglomerated by sonification. De-agglomeration can be measured by taking a particle size distribution before and after sonification. After sonification, the size distribution should shift, thereby showing smaller particles. Thereafter, the slurry (when at rest) will tend to once again (stage 1) agglomerate.
The stage 1 agglomerates of the present invention are stable, and stable is intended to mean that the stage 1 agglomerates will resist stage 2 agglomeration for a period of at least 3 months. Preferably, less than 15 percent (by volume) of the stage 1 agglomerates will become stage 2 agglomerates when at rest for 3 months, more preferably less than 10 percent, yet more preferably less than 5 percent, yet more preferably less than 2 percent and yet more preferably less than 1 percent of the stage 1 agglomerates will become stage 2 agglomerates when at rest for 3 months.
Applicants have found that particles capable of providing stage 1 agglomeration (without also causing stage 2 agglomeration) provide a superior metal polishing slurry relative to slurries having a force sufficient to repel and overcome the van der Waals forces between the particles, e.g., do not agglomerate. Hence, Applicants have found that agglomeration is not the problem, but rather bridging (e.g., the formation of hard, dense sediment) is what harms slurry performance. Not only can agglomerating slurries function well as a metal polishing slurry, but indeed, a slurry system which enables particle agglomeration without bridging has been found to surprisingly provide improved polishing performance, particularly in the polishing of metal layers as part of the manufacture of semiconductor devices.
Applicants have found that improved planarization length and diminished dishing is possible with polishing slurries having stage 1 agglomerates. It is believed that stage 1 agglomerates will tend to settle into indentations in the substrate being polished, but will readily de-agglomerate or otherwise wear (without scratching) during the polishing operation, thereby protecting the indentation from further polishing erosion by preventing the polishing operation from entering (and then increasing) the indentation.
Part of this advantageous behavior (of filling in indentations) is believed to be due to the lack of (or a diminished amount of) a charge density around the particles. After polishing, the stage 1 agglomerates are easily washed away, particularly when the ionic strength of the aqueous medium is changed to thereby re-normalize the electrostatic charge on the particles, thereby making the particles less compatible with being located within the indentation. Hence the slurries of the present invention provide stage 1 agglomerates that readily fill indentations when is it advantageous to do so and are readily removed from the polished surface when it is advantageous to do so. This type of selective filling and unfilling of indentations is very unique and allows superior planarization polishing over conventional metal slurries.
Stage 1 particle agglomerates generally have diminished electrostatic layer(s) and such particles tend to provide improved polishing by better interacting with the surface chemistry of a polishing substrate. Furthermore, the ionic species which inhibit or destroy electrostatic layers (around the particles) are preferably selected to provide other polishing advantages. For example, an ionic species can be used to buffer pH, provide a complexing agent to other ions in suspension (inhibit re-deposition), and/or provide selectivity (certain ionic species may protect portions of a surface, so that other portions of the surface will exhibit a higher removal rate).
Ionic species such as acids, bases, salts, complexing agents, surfactants, electrolytes, and the like are all well known, and indeed, such ionic species are, broadly speaking, known for chemical mechanical polishing. However, it has been surprisingly discovered that when an appropriate level of ionic species is introduced into a polishing slurry (e.g., where the aqueous medium has a sufficiently ionic strength), polishing performance of the slurry can be improved and unwanted particle bridging can be substantially inhibited. In a preferred embodiment, the slurry""s total ion concentration is greater than 0.001 molar, more preferably greater than 0.01 molar, yet more preferably greater than 0.05 molar, yet more preferably greater than 0.1 molar, yet more preferably greater than about 0.2 molar and yet more preferably greater than about 0.5 molar. In a preferred embodiment, the total ion concentration is also less than 2 molar, more preferably less than 1 molar. In a preferred embodiment, the metal oxide particles also have a maximum zeta potential greater than about plus or minus 0.10 millivolts in an aqueous medium having an ion concentration of less than 0.001 molar.
The slurries of the present invention are particularly well suited for polishing operations having high polishing surface speeds. For example, many newer polishing machines are polishing at increasingly higher revolutions per minute, and the slurries of the present invention are particularly well suited for such high speed polishing (e.g., rotary polishing speeds greater than 100 rpm, greater than 150 rpm and/or greater than 200 rpm).
The slurries of the present invention are also well suited for polishing dielectrics (silica), including low k dielectrics, such as porous silica, or organic low k dielectrics, such as fluoro polymers or copolymers.
In a preferred embodiment, any stage 1 agglomerate transported onto the polishing interface or region will de-agglomerate or otherwise wear, rather than scratch or otherwise cause defects on the surface being polished.
In another embodiment of the present invention, the ionic strength of the slurry is adjusted after the polishing operation, thereby restoring (or increasing) the electrostatic layers around each particle. This in turn will generally cause the particles to be more easily cleaned or otherwise removed from the polished surface.
The present invention is particularly advantageous for fumed particles, since fumed particles generally have more potential sites for stage 2 agglomeration.
The slurries of the present invention comprise constituents which not only inhibit or prevent stage 2 agglomeration, but also, are sufficiently benign to the metal surface being polished to have a static metal etch rate of less than 50 Angstroms per minute, more preferably less than 40 Angstroms per minute, yet more preferably less than 30 Angstroms per minute, yet more preferably less than 20 Angstroms per minute and yet more preferably less than 10 Angstroms per minute (up to and including 0 Angstroms per minute).
The polishing compositions of the present invention can be created before or during the polishing operation. If created during a polishing operation, the polishing fluid can be introduced into a polishing interface and then some or all of the particles can be introduced into the polishing interface by means of particle release from a polishing pad. For example, a polishing pad type substrate comprising particles is described in U.S. Pat. No. 5,692,950 to Rutherford, et al. (which is hereby incorporated into this specification by reference), and in the use of such a polishing substrate during polishing, the polishing substrate will release particles, into the polishing interface which also contains a polishing fluid. As the polishing fluid and particles mix (in accordance with the present invention), they become a metastable polishing slurry, whereby the slurry will be capable of forming stage 1 agglomerates without substantial formation of stage 2 agglomerates.